There is no mass exodus out of SL either. Just a long slow movement. My own experience is that SLers generally move to a closed grid, such as InWorldz or Kitely. A factor would likely be whether a grid has a functioning economy or not.
Or they try something much different, such as Unity. (I have to admit to not having much familiarity with anything other than OpenSim & AuroraSim).
Looking at the graphs provided by Maria, it is clear that there is no one-to-one correlation. I agree that much of OpenSim's region growth is due to single individuals running multiple or megaregions.
Yet, I believe that there is a lot of word-of-mouth going on, too. I know that I still drive my RL friends nuts telling them about all the fun I am having :-D